A typical motor control scheme employs three Hall sensors 2 to provide position feedback to motor control circuitry, as illustrated in FIG. 1. This scheme is employed, e.g., in the TAPE250 tape drive available from Iomega Corporation, the assignee of the invention disclosed herein. Motor control signals, produced by control circuitry 4, are produced in one of six states, denoted A, B, C, D, E, and F, depending on the respective Hall sensor signals H.sub.1, H.sub.2, H.sub.3. Each control signal state defines a particular combination of drive signals, denoted "PHASE A", "PHASE B", and "PHASE C" in FIG. 1. These drive signals drive windings on the stator 8. Each control signal state (or combination of drive signals) attracts the rotor magnets 6 to a specific stable physical position corresponding to that state. For example, if control state A were asserted and the rotor position permitted to stabilize, the rotor would reach a corresponding position "A".
The motor states are separated by a prescribed amount of angular rotation (e.g., 10 degrees). The three Hall sensor signals "H.sub.1," "H.sub.2," "H.sub.3 " provide information (feedback) about the rotor position. The Hall sensors 2 are aligned so that the sensor signals change state at the stable rotor positions A through F. As the motor control signals cycle through control states A through F, the three Hall signals "H.sub.1," "H.sub.2,"]"H.sub.3 " cycle through six position states, denoted 1 through 6, as follows (note that, although only six position states are used in this example, there are actually eight position states available with three sensors):
______________________________________ Motor Control State .linevert split.A.linevert split.B.linevert split.C.linevert split.D.linev ert split.E.linevert split.F.linevert split.A.linevert split.B.linevert split.C.linevert split.D.linevert split.E.linevert split.F.linevert split.-- 1.linevert split.2.linevert split.3.linevert split.4.linevert split.5.line vert split.6.linevert split.1.linevert split.2.linevert split.3.linevert split.4.linevert split.5.linevert split.6.linevert split.1- Hall State ______________________________________
In the above diagram, the Hall state changes precisely at the static stable rotor position. Therefore, the midpoint of each Hall state is halfway between two static stable points and each static stable point is at the transition between two Hall states.
In the TAPE250 tape drive, the control logic operates as follows: If the Hall sensors are in state 1, control state B is asserted to advance the rotor towards state 2. Alternatively, to rotate the rotor in the opposite direction, control state E is asserted to move the rotor toward state 6. The Hall state indicates the rotor's position to within a range (e.g., 10 degrees). A control state is asserted corresponding to a static position leading 15 degrees from the approximate rotor position indicated by the Hall sensors. The following table indicates the control state asserted for each Hall state and the desired direction of rotation (note that, in this example, right is clockwise and left is counter-clockwise).
______________________________________ Control for Desired Direction of Motion Hall State + (right) - (left) ______________________________________ 1 B E 2 C F 3 D A 4 E B 5 F C 6 A D ______________________________________
In the highly competitive computer industry, the use of multiple sensors to provide positional feedback adds to the production cost of tape and disk drive units. While Hall sensors are relatively inexpensive on a per-unit basis, the requirement of multiple position sensors for each tape or disk drive can add hundreds of thousands or even millions of dollars to the manufacturer's total production costs, depending on the number of units produced. Thus, the need for multiple position feedback sensors is a disadvantage to manufacturers of tape drives, disk drives, and other devices requiring motor control.